As weird as it may sound... I think it was Nintendo Land. Other games were mostly (bad) ports and New Super Mario Bros. U, which was decent, but so similar to its predecessors... not to mention the lack of ambition (bad graphics and the same old OST).
So yeah, even if Nintendo Land ended up being a decent game at best, I still think it was the best available at Wii U's launch. If only it had online...
From third parties... I would definitely pick ZombiU. It was badly executed and too short, but the concept was great, the gamepad use was phenomenal and it was something fresh.
Bet with Teeqoz for 2 weeks of avatar and sig control that Super Mario Odyssey would ship more than 7m on its first 2 months. The game shipped 9.07m, so I won








